The SNAP receptor (SNARE) complex is a core complex specialized for synaptic vesicle exocytosis and the binding of SNAP to the complex is an essential step for neurotransmitter release.Complexin I and II were identified as SNARE complex-associated proteins and have been shown to compete with alpha-SNAP for binding to the SNARE complex. This suggested that complexins may modulate the dynamics of neurotransmitter release. To examine this possibility, we tried presynaptic injection of complexin II and the anti-complexin II antibody into the neurons of Aplysia buccal ganglia.The results showed that complexin II inhibited neurotransmission and conversely, the antibody stimulated it.Next we generated complexin II knockout mice and performed electrophysiological analysis on the hippocampal slice. The complexin lI-deficient mice were viable, fertile, and appear normal. Electrophysiological recordings in the mutant hippocampus showed that ordinary synaptic transmission and paired-pulse facilitation, a form of short-term synaptic plasticity, were normal. However, long-term potentiation (LTP) in both CAl and CA3 regions was impaired, suggesting that complexin II may not be essential for synaptic vesicle exocytosis, but has some roles in the establishment of hippocam pal LTP.